The ink jet printing system suitably applicable to industrial and home use printers using the above aqueous ink is a system that transforms input image data into an output image by means of a liquid ink. A technique for maintaining performance of an ink ejecting print head is important from the standpoint of reliability of a printing operation and is employed for the following reasons.
(a) An ink ejection print head ejects ink directly onto a print medium from its fine nozzles (which generally refer to ink ejection openings, liquid paths communicating with the openings and elements that generate energy for ejecting ink). The ejected ink, when it lands on the print medium, may bounce off it. Further, during the ink ejection operation, fine ink droplets (satellite or secondary droplets) may be ejected from the print head in addition to the main ink droplets intended to form an image and may drift in the atmosphere. These fine droplets form ink mist which in turn may adhere to around the ink ejection openings of the print head. The ink that settled around the ejection openings may pull the main ink droplets as they are ejected, deflecting a direction of the ejecting droplets, i.e., interfering with the main ink droplets flying in straight line.(b) The print head generally has a plurality of nozzles arrayed therein for the purpose of increasing the printing speed and resolution. Depending on input image data, there may be nozzles during the printing operation that are not used for ink ejection. From the ejection openings of such nozzles an ink solvent evaporates to increase a viscosity of ink in these nozzles. Therefore, when the nozzles in question are used thereafter, the print head may not be able to eject ink stably when applied a normal ink ejection energy, resulting in an ejection failure. Further, if bubbles exist in ink reservoirs inside the print head, i.e., in the liquid paths on the inner side of the nozzles or in a common chamber communicating with the liquid paths, a gas that has passed through the ejection openings and the interior of a member forming the print head may get trapped in the bubbles which in turn grow in size. The bubbles may also inflate due to a temperature increase during the printing operation. These will interfere with normal ink supply from an ink source, causing an ink ejection failure.
As a maintenance technique to solve the problems described in (a) and (b), the following is employed in the ink jet printing apparatus.
(A) A surface of the print head on which ejection openings are formed (hereinafter referred to as an ejection face) is wiped with a wiping member formed of an elastic material such as rubber, at a predetermined timing to remove adhering ink (this operation is called a wiping).(B) Aside from the ink ejection for forming an image on a print medium, a predetermined amount of ink is ejected to discharge ink of an increased viscosity according to the time in which ink ejection is not performed and an environment (this operation is called a preliminary ejection). Also, a suction force is applied to the ejection face at a predetermined timing, as by operating a pump, to forcibly suck out ink from an inner part of the nozzles through the ejection openings (this operation is called a suction-based recovery). The ejected or sucked-out ink (hereinafter called waste ink) is received in a cap which can oppose the ejection face or which can form a hermetic space around the ejection openings. The waste ink is then moved to and retained in an absorbent installed at a predetermined location.
Among aqueous inks commercially available in recent years (that may or may not include colorants such as dye, pigment and dye-pigment mixture), there is an ink containing a pigment as the colorant (pigment-based ink) which is developed to meet a demand for an increased image fastness. Compared with an ink containing dye (dye-based ink), the pigment-based ink is less capable of re-dispersing a colorant after evaporation and has a characteristic that a high molecular compound used to disperse the pigment in a solvent is easily adsorbed to the ejection face.
In the printing apparatus using the pigment-based ink, therefore, simply wiping the ejection face of the print head may leave ink, whose viscosity was increased by the evaporation of ink solvent, on the ejection face. As a result, the problem (a) may not be alleviated. The pigment-based ink is made by dispersing a solid colorant in water as by using a dispersant or introducing a functional group to a pigment surface. Thus, a dry substance of the pigment-based ink whose water content has dried on the ejection face has a greater damaging effect on the ejection face than a dry substance of the dye-based ink which has the colorant itself dissolved at a molecular level. The pigment-based ink is also characterized in that the high molecular compound used to disperse the pigment in a solvent is easily adsorbed to the ejection face. This problem also occurs in other than the pigment-based ink whenever high molecular compounds exist in the ink as a result of adding a reaction liquid to the ink for adjustment of ink viscosity, for improvement of lightfastness or for other purposes.
To cope with these problems, Japanese Patent Application Laid-open Nos. 10-138503 and 2000-203037 disclose a technique that applies liquid for a head (hereinafter called ‘head liquid’) to the ejection face during the print head wiping operation. This alleviates a wear of the wiper, dissolves ink residues adhering to the print head to remove them, and forms a thin film of head liquid on the print head to prevent foreign matters from adhering to the print head. As a result, the wiping performance is improved. The head liquid used for wiping is stored in the interior of the printer body. Further, the Japanese Patent Application Laid-open Nos. 10-138503 and 2000-203037 disclose a step of cleaning the ejection face by moving the wiper relative to the ejection face and a step of supplying, before the cleaning step, a nonvolatile solvent as the head liquid to the ejection face. These documents, however, offer a very limited description on the nonvolatile solvent. That is, the Japanese Patent Application Laid-open Nos. 10-138503 and 2000-203037 only disclose polyethyleneglycol (PEG) with a molecular weight of 200-600 and polyethyleneglycol (PEG300) with a molecular weight of 300, respectively.
As to the use of cooling means in the ink jet printing technology, the following is available. Japanese Patent Application Laid-open No. 54-51837 discloses a technique that forms a bubble by a nucleate boiling and cools to collapse the bubble in a very short time or in microseconds by using a Peltier device. Even if a balance between heat dissipation and cooling and products on the market are observed, it is thought that such a Peltier device has yet to come out of a primordial stage of idea. On the other hand, Japanese Patent Application Laid-open No. 2000-276214 discloses a Peltier device provided on the outside of a cap member with a seal portion, which is adapted to open to the nozzle face (ejection face) of the print head, for the purpose of preventing a possible clogging of a group of nozzles arrayed on the print head in the ink jet printing apparatus. The Japanese Patent Application Laid-open No. 2000-276214 improves the humidity inside the cap to prevent the nozzle clogging. This technology, however, cannot improve the humidity in the cap when the cooling operation is performed with the nozzles capped. Conversely, if the cap is cooled before the capping operation, no desirable response can be expected nor can an absolute humidity enough to prevent head clogging be obtained because the cap has a communication tube for discharging ink.
In another field separate from the ink jet printing, Japanese Utility Model Registration No. 2547929 discloses a construction in which a Peltier device is used to cool to −15° C. to −20° C. a pen point which comes into contact with a print medium that produces color upon cooling. The Japanese Utility Model Registration No. 2547929 has a description that any condensed dew layer or water drop adhering to the pen point will degrade the quality of a thermally transformable object, suggesting that a dew condensation should be avoided.
The print head cleaning device described in the Japanese Patent Application Laid-open No. 10-138503, however, must be provided with a reservoir for storing a sufficient amount of nonvolatile solvent to be applied to the wiper during the wiping operation that is performed until the end of life of the printer body. This poses a problem of an increased size of the printer. Although the size of the printer may be reduced by adopting a construction that the nonvolatile solvent is periodically replenished, the user is required to perform the cumbersome processing. If a processing liquid available in the market is used, the running cost can increase. Further, depending on the environment in which the printer is used, the nonvolatile solvent may become dry or wet, changing its property, which in turn results in variations in the amount of solvent applied to the wiper and therefore a possibility of a desired wiping performance failing to be produced.
Further, the Japanese Patent Application Laid-open No. 10-138503 is primarily concerned with the application of the nonvolatile solvent to the wiper and is not aware of the problem that a waste ink introducing portion of an absorbent is blocked by a viscous ink or solidified ink and that this prevents the waste ink from spreading into the interior of the absorbent, resulting in an overflow of the waste ink. Naturally, a solution to this problem is not suggested.
As for the problem (b) described above, as a waste ink is introduced into the absorbent, viscous ink and, in a worse case, solidified ink accumulate at the waste ink introducing portion of the absorber. Therefore, the waste ink introducing portion may be blocked, preventing the infiltration of waste ink into the interior of the absorbent, which in turn may result in an overflow of waste ink.